1. Field of the Invention
The present invention relates to a sheet finisher mounted on or operatively connected to a copier, printer or similar image forming apparatus for folding, sorting, stacking, stapling, center-stapling and binding, folding or otherwise finishing a sheet or a sheet stack, and an image forming system consisting of the sheet finisher and image forming apparatus.
2. Description of the Background Art
A sheet finisher positioned at the downstream side of an image forming apparatus for stapling or otherwise finishing a sheet stack is well known in the art. To meet the increasing demand for multiple functions, a sheet finisher having a center-stapling capability in addition to the conventional edge-stapling capability has recently been proposed. Further, a sheet finisher with a center-folding capability in addition to the center-stapling capability has been proposed to fold a center-stapled sheet stack at the center for thereby producing a pamphlet.
A sheet finisher with the binding capability mentioned above uses, in many cases, one or more pairs of fold rollers to fold a sheet stack. In this type of sheet finisher, a flat fold plate is caused to contact the stapled position of a sheet stack and push it into the nip of the fold roller pair, thereby folding the sheet stack. When use is made of, e.g., a first and a second fold roller pair, after the first roller pair has folded a sheet stack, the second roller pair presses the resulting fold of the sheet stack for thereby reinforcing it.
A problem with the above configuration, causing a fold roller pair to fold a sheet stack, is that the pressing force of the roller pair cannot be sufficiently transferred to a sheet stack because the entire width of a sheet stack passes the nip of the roller pair in an extremely short period of time. To solve this problem, Japanese Patent Laid-Open Publication Nos. 9-183566 and 9-183567, for example, propose to control the rotation speed of a fold roller pair for thereby enhancing folding quality. However, a pressing time available with a single fold roller pair is limited because the nip width of the roller pair is extremely small. Further, the above proposal reduces productivity. In light of this, Japanese Patent Laid-Open Publication No. 2000-143088 teaches the use of two fold roller pairs, which seems to be advantageous over the use of a single fold roller pair from the folding quality standpoint.
In any case, however, a period of time over which a sheet stack is pressed by the nip of a fold roller pair is short because the axis of each fold roller extends perpendicularly to a direction of sheet conveyance. This, coupled with the fact that the pressure of the fold roller pair, pressing the entire portion of a sheet stack to be folded, is scattered, prevents the sheet stack from being sharply folded.
Usually, a person folds a sheet stack by nipping the portion of the sheet stack to be folded with fingers and can therefore fold it with a relatively weak force. This is presumably because a sheet stack is not folded over the entire width at a time, but is folded part by part, so that a force to act on each part for a unit length increases. Taking this into account, Japanese Patent Laid-Open Publication No. 62-16987 proposes to surely fold a sheet stack by causing a roller to roll on the sheet stack in the direction perpendicular to the direction of sheet conveyance, i.e., parallel to the direction of a fold. More specifically, in a folding device configured to fold a sheet stack by conveying the sheet stack via the nip of a roller pair, a reinforce roller is positioned at the downstream side of the above roller pair and movable substantially perpendicularly to the direction of sheet conveyance for again pressing the fold of the sheet stack folded by the roller pair. The reinforce roller reinforces the fold of a sheet stack by being driven by a ball screw in the direction perpendicular to the direction of sheet conveyance.
In the configuration taught in Laid-Open Publication No. 62-16987 mentioned above, the reinforce roller presses the fold of a sheet stack in the direction perpendicular to the direction of sheet conveyance, so that load concentrates on one portion of the fold. In addition, the reinforce roller rolls on the fold of a sheet stack while exerting pressure on the entire fold of the sheet stack. The reinforce roller can therefore easily make the fold of the sheet stack sharper. However, the reinforce roller scheme taught in the above document has the following problems (1) through (7) when the sheet stack is thick.
(1) When the reinforce roller rolls on the fold of the sheet stack, it is likely that the roller sinks into the sheet stack and therefore moves on the fold without rotating, so that the image surface of a sheet is rubbed and smeared.
(2) The reinforce roller, fully pressed the fold of the sheet stack, comes down from the fold onto a lower guide plate. At this instant, the reinforce roller is apt to produce noise due to an impact.
(3) If a movable support member, supporting the reinforce roller, tilts while the roller is in movement, then the roller itself tilts with the result that the pressing force of the roller expected to act on the fold escapes. This prevents the reinforce roller from neatly reinforcing the fold.
(4) When a belt, which transfers a driving force to the reinforce roller, twists due to the tilt of the reinforce roller, it is likely that the durability of the belt is reduced or the belt slips out.
(5) If a guide member, which guides the movable support member, bends due to the pressing force of the reinforce roller while the roller is in movement, then the pressing force of the roller, acting on the fold, escapes, again preventing the roller from neatly reinforcing the fold.
(6) If a position where the reinforce roller and lower guide plate contact each other is different in level or height from the nip of a folding device located upstream of the roller, then it is likely that the sheet stack is formed with two folds.
(7) If the level at which the reinforce roller and lower guide plate contact each other varies in accordance with the position of the roller being moved, then the fold of the sheet stack is apt to be oblique.
Further, the reinforce roller scheme of Laid-Open Publication No. 62-16987 has the following problems (8) through (10) unsolved.
(8) When the number of sheets stapled together is small, the interval between consecutive sheet stacks is short, making a period of time necessary for the reinforce roller to press each sheet stack unavailable.
(9) When the number of sheets stapled together is large, each sheet stack cannot be sufficiently folded unless the reinforce roller presses the sheet stack a larger number of times or over a longer period of time.
(10) It is difficult to reduce the folding time of the reinforce roller while enhancing the durability of the roller.
When a roller pair is used to reinforce the fold of a sheet stack while conveying it, the roller pair is generally formed of an elastic material because it must exert a conveying force. Therefore, even when the sheet stack is relatively thick, noise to be produced when the trailing edge of the sheet stack leaves the nip of the roller pair is low and unnoticeable. By contrast, the reinforce roller, movable perpendicularly to the direction of sheet conveyance while rolling on the fold of a sheet stack, does not have to exert a conveying force, so that the reinforce roller and lower guide plate both can be formed of a hard material for the reinforcing effect. However, the reinforce roller, formed of a hard material, produces high, noticeable noise when coming down from the sheet stack onto the lower guide plate. The construction of Laid-Open Publication No. 62-16987 indicates that this problem is not addressed to.
On the other hand, if a jam occurs when the reinforce roller is moving in the direction perpendicular to the direction of sheet conveyance, then it is difficult to deal with the jam because of a relation between the direction of the nip and the direction of sheet conveyance. In the case of a roller pair, a person may forcibly pull out the jamming sheet stack or a rotatable knob may be arranged by a relatively simple, low cost method. However, when the reinforce roller stops moving halfway on the sheet stack, forcibly pulling out the sheet stack by hand is apt to damage the machine or the rotatable knob makes the configuration sophisticated.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-2426, 2001-10759, 2001-19269 and 2002-145516.